This invention relates, in general, to surface acoustic wave (SAW) devices, and more specifically, to acoustic charge transport (ACT) devices.
ACT is a relatively new technology relating to SAW devices. ACTs can perform many functions such as delay and transversal filtering, and are formed on Gallium Arsenide (GaAs) substrates to take advantage of the high processing speed of GaAs.
As explained in U.S. Pat. No. 4,633,285, issued Dec. 30, 1986 to Billy Jo Hunsinger and assigned to University of Illinois, an ACT comprises a buried channel formed by placing a piezoelectric semiconductor material between confining layers of doped GaAs. A SAW is then generated within the piezoelectric layer. As the SAW propagates through the piezoelectric layer, charge representing an input signal is injected into the wells of the SAW from which majority carriers have been depleted. The charge representing the signal is carried by the SAW below the electrodes which are capacitively coupled to the piezoelectric layer. The electrodes can sense the magnitude of the charge in each SAW well without actually extracting charge from the signal. This process is termed non-destructive sensing since the capacitively extracted charge magnitude does not deplete the charge in the SAW well.
Each SAW has an associated electrical potential which allows the charge representing the input signal to be carried. The potential of the SAW has an impact on the amplitude of the input signal. The signal, or associated charge, is very non-viscous. In other words, the direction of motion of the charge can change almost instantaneously. The direction of motion of the charge is determined by the electrical potential between the input contact and the SAW potential curve. When the SAW potential is negative with respect to the input contact, charge will flow towards the SAW. When the potential difference is positive, charge will flow back to the contact. At a potential difference of zero, no charge will flow, and an effective "break" between the contact and the SAW will occur.
Varying the input charge magnitude to a constant SAW potential causes the breaking point between the input contact and the SAW to vary. This variance results in samples of the signal to be taken other than at the exact desired periodic sample time. When the signal is sampled other than at the appropriate sample time, the signal, as read by a subsequent filter, becomes distorted. This distortion has been found in all ACT devices to date. With the need for clear and precise communication devices, this distortion is unacceptable.